- Use `exec_wait` only after `exec` returns `Script running with cell ID ...`.
- `cell_id` identifies the running `exec` cell to resume.
- `yield_time_ms` controls how long to wait for more output before yielding again. If omitted, `exec_wait` uses its default wait timeout.
- `max_tokens` limits how much new output this wait call returns.
- `terminate: true` stops the running cell instead of waiting for more output.
- `exec_wait` returns only the new output since the last yield, or the final completion or termination result for that cell.
- If the cell is still running, `exec_wait` may yield again with the same `cell_id`.
- If the cell has already finished, `exec_wait` returns the completed result and closes the cell.
